1. Field of the Invention
This invention relates generally to a laser-plasma, extreme ultraviolet (EUV) radiation source and, more particularly, to a laser-plasma EUV radiation source having a target material delivery system that employs a droplet generator in combination with one or more of a drift tube, accelerator chamber and vapor extractor to provide tightly-controlled target droplets.
2. Discussion of the Related Art
Microelectronic integrated circuits are typically patterned on a substrate by a photolithography process, well known to those skilled in the art, where the circuit elements are defined by a light beam propagating through a mask. As the state of the art of the photolithography process and integrated circuit architecture becomes more developed, the circuit elements become smaller and more closely spaced together. As the circuit elements become smaller, it is necessary to employ photolithography light sources that generate light beams having shorter wavelengths and higher frequencies. In other words, the resolution of the photolithography process increases as the wavelength of the light source decreases to allow smaller integrated circuit elements to be defined. The current state of the art for photolithography light sources generate light in the extreme ultraviolet (EUV) or soft x-ray wavelengths (131-14 nm).
U.S. Pat. No. 6,324,256, entitled xe2x80x9cLiquid Sprays as a Target for a Laser-Plasma Extreme Ultraviolet Light Source,xe2x80x9d and assigned to the assignee of this application, discloses a laser-plasma, EUV radiation source for a photolithography system that employs a liquid, such as xenon, as the target material for generating the laser plasma. A xenon target material provides the desirable EUV wavelengths, and the resulting evaporated xenon gas is chemically inert and is easily pumped out by the source vacuum system. Other liquids and gases, such as argon and krypton, and combinations of liquids and gases, are also available for the laser target material to generate EUV radiation.
The EUV radiation source employs a source nozzle that generates a stream of target droplets. The droplet stream is created by forcing a liquid target material through an orifice (50-100 microns diameter), and perturbing the flow by voltage pulses from an excitation source, such as a piezoelectric transducer, attached to a nozzle delivery tube. Typically, the droplets are produced at a rate (10-100 kHz) defined by the Rayleigh instability break-up frequency of a continuous flow stream for the particular orifice diameter.
To meet the EUV power and dose control requirements for next generation commercial semiconductors manufactured using EUV photolithography, the laser beam source must be pulsed at a high rate, typically 5-10 kHz. It therefore becomes necessary to supply high-density droplet targets having a quick recovery of the droplet stream between laser pulses, such that all laser pulses interact with target droplets under optimum conditions. This requires a droplet generator which produces droplets with precisely controlled size, speed and trajectory.
Various techniques have been investigated in the art for delivering liquid or solid xenon to the target location at the desirable delivery rate and having the desirable recovery time. These techniques include condensing supersonic jets, liquid sprays, continuous liquid streams and liquid/frozen droplets. As an example of this last technique, commercial droplet generators, such as inkjet printer heads, have been investigated for generating liquid droplets of different sizes that can be used in EUV sources.
The use of known droplet generators for providing a low temperature, high-volatility, low surface tension, low-viscosity fluid, such as liquid xenon, in combination with the need to inject the droplets into a vacuum provides significant design concerns. For example, because the target material is a gas at room temperature and pressure, the material must be cooled to form the liquid. Thus, it is important to prevent the liquid droplets from immediately flash boiling and disintegrating as they are emitted from the nozzle into the source vacuum. Also, because the cooled liquid droplets that do not immediately flash boil will evaporate and freeze as they travel through the source environment, the source parameters must be tightly controlled to insure the resulting size and consistency of the droplets at the target location is correct. Additionally, the speed, spacing and frequency of production of the droplets must be controlled.
In accordance with the teachings of the present invention, a target material delivery system, or nozzle, for an EUV radiation source is disclosed. The nozzle includes a target material chamber having an orifice through which droplets of a liquid target material are emitted. The size of the orifice and the droplet generation frequency is provided so that the droplets have a predetermined size, speed and spacing therebetween. In one embodiment, the droplets emitted from the target chamber are mixed with a carrier gas and the mixture of the droplets and carrier gas is directed into a drift tube. The carrier gas provides a pressure in the drift tube above the pressure of the source vacuum chamber to prevent the droplets from flash boiling and disintegrating. The drift tube allows the droplets to evaporate and freeze as they travel to become the desired size and consistency for EUV generation.
In one embodiment, the droplets are directed through an accelerator chamber from the drift tube where the speed of the droplets is increased to control the spacing therebetween. A vapor extractor can be provided relative to an exit end of the drift tube or accelerator chamber that separates the carrier gas and the vapor resulting from droplet evaporation so that these by-products are not significantly present at the laser focus area, and therefore do not absorb the EUV radiation that is generated.
Additional objects, advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.